Additive-Free Isomerization of Allylic Alcohols to Ketones with a Cobalt PNP Pincer Catalyst

Article abstract of DOI:10.1002/chem.201901148

Catalytic isomerization of allylic alcohols in ethanol as a green solvent was achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobalt PNP pincer complex substituted with phenyl groups on the phosphorus atoms appeared to be the most active. High rates were obtained at 120 °C, even though the addition of one equivalent of base increases the speed of the reaction drastically. Although some evidence was obtained supporting a dehydrogenation–hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double-bond isomerization through alkene insertion–elimination.

Full text of DOI:10.1002/chem.201901148

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Accepted Article
Title: Additive Free Isomerization of Allylic Alcohols to Ketones with a
Cobalt PNP Pincer Catalyst
Authors: Brian Spiegelberg, Andrea Dell'Acqua, Tian Xia, Anke
Spannenberg, Sergey Tin, Sandra Hinze, and Johannes
Gerardus de Vries
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To be cited as: Chem. Eur. J. 10.1002/chem.201901148
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Additive Free Isomerization of Allylic Alcohols to Ketones with a
Cobalt PNP Pincer Catalyst
Brian Spiegelberg, Andrea Dell´Acqua, Tian Xia, Anke Spannenberg, Sergey Tin, Sandra Hinze,
Johannes G. de Vries*
Abstract: Catalytic isomerization of allylic alcohols in ethanol as
green solvent is achieved using air and moisture stable cobalt (II)
complexes in the absence of any additives. Under mild conditions
the cobalt PNP pincer complex substituted with phenyl groups on
the phosphorus atoms appears to be the most active. High rates
were obtained at 120 °C even though the addition of one equiva-
lent of base increases the speed of the reaction drastically. Alt-
hough we obtained some evidence supporting a dehydrogenation-
hydrogenation mechanism, we gathered evidence that this is not
the major mechanism. Instead the cobalt hydride complex formed
by dehydrogenation of ethanol is capable of double bond isomeri-
zation via alkene insertion elimination.
thermore, Liu and co-workers recently reported impressive
findings about pincer cobalt complexes that are suitable cata-
lysts for regioselective olefin isomerization by using ammonia
borane as activation source.[13] Although many catalytic reac-
tions have been performed with pincer cobalt catalysts, isom-
erization reactions of allylic alcohols with this type of complex-
es are still unknown.
Isomerization reactions comply with the principles of green
chemistry.[1] The catalytic isomerization of allylic alcohols into
the saturated carbonyl compounds is an elegant synthetic
process that eliminates the more conventional two-step oxida-
tion and reduction pathway.[2] During the last half century,
many precious metal catalysts for allylic alcohol isomerization,
based on Ir, Ru, Rh or Pd have been developed.[2, 3] Some of
these even work well at ambient temperature.[4] Although
noble metals play an indispensable role in catalysis, they are
rather expensive, less abundant and generally not used in the
last step of drug synthesis. Thus, catalysts based on cheap
metals like iron[5] and nickel[6 have been used for the isomeri-
zation of allylic alcohols recently. Nevertheless, the require-
ment of either activated substrates[5b] or the need for activa-
tion of the catalyst make those systems less desirable. Re-
cently, reports have appeared on the sole use of base as
catalyst for the isomerization of allylic alcohols to the corre-
sponding ketones.[7] To the best of our knowledge, there is
only one report on the use of a cobalt catalyst (HCo(CO)4) that
can promote the isomerization of an allylic alcohol to an alde-
hyde or a ketone, but harsh conditions including the use of
toxic carbon monoxide were required to achieve low conver-
sions and selectivities towards the desired products.[8] Recent-
ly, pincer cobalt complexes have been discovered to be effec-
Figure 1. PNP-Co complexes applied to the isomerization of allylic alcohols
in this work.
Herein, we report the first cobalt(II) catalyzed isomeriza-
tion of allylic alcohols to ketones in ethanol as solvent. We
have tested a number of cobalt PNP pincer complexes having
different substituents on the phosphorus atoms. It was
demonstrated, that the reaction works in the presence of a
base, as well as in the absence of it or any other additives.
The present work describes the successful application of
(PNHPPh)CoCl2 (Co-4), a catalyst hitherto known for the stere-
ospecific polymerization of 1,3-butadiene[17] and the reduction
of carboxylic acid esters and nitriles.[18]
Initial studies were carried out under exploitation of differ-
ent substituted cobalt pincer catalysts in diverse solvents (see
19]
ESI). The reported complexes[18,
are easily synthesized
upon the addition of the respective ligands to THF or ethanol
solutions of cobalt(II) precursors (see ESI). It was assumed
that a base is required since deprotonation of the cobalt com-
plexes results in formation of the active amido species,[11a]
which should be capable of alcohol dehydrogenation. Ten
different solvents were tested with various polarities and
proton-donor capabilities. The alcohols turned out to be the
most promising solvents, resulting in high conversions of our
model substrate oct-1-en-3-ol (1) towards the desired product
3-octanone (2). Taking efficiency and environmental impact[20]
into account, ethanol was selected as reaction medium for
further experiments. Although different bases were examined,
potassium tert-butoxide, chosen in the preliminary studies,
maintains the best among the others (see ESI). However, to
accommodate base-sensitive substrates and for reasons of
waste reduction we also examined the possibility of base-free
catalysis.
11]
tive catalysts[9] for hydrogenation,[10] dehydrogenation,[10g,
transfer hydrogenation,[12] hydrosilylations/hydroborations,[13]
alkene isomerization[14] and N2-activation[15]. Hanson and co-
workers reported a cationic cobalt(II) alkyl complex, which was
applied for the hydrogenation of aldehydes, ketones and
imines, as well as for the isomerization of alkenes.[16] Fur-
[*]
B. Spiegelberg, A. Dell´Acqua, Dr. T. Xia, Dr. A. Spannenberg Dr. S.
Tin, Dr. S. Hinze, Prof. Dr. J. G. de Vries
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-
Einstein-Strasse 29a, 18059 Rostock
E-mail: Johannes.deVries@catalysis.de
Homepage: https://www.catalysis.de/en/research/catalysis-with-
renewable-resources/catalysis-with-renewables-platform-chemicals/
For a first screening (Figure 2) all cobalt pincer complexes
(Co-1 to Co-6) were tested for catalytic isomerization of oct-1-
en-3-ol (1) with the aid of KOtBu. While in case of Co-2 and
Supporting information for this article is given via a link at the end of
the document.
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Attempts to reduce the reaction temperature resulted in
lower yields. The reaction did not proceed at 60 °C or lower
temperatures (see ESI). Likely, the poor solubility of the cobalt
complex in ethanol is the crucial factor that limits the catalytic
[17]
performance.
However, above 70 °C the catalyst is fully
soluble. On the positive side, this poor solubility allowed the
easy separation of the catalyst from the reaction mixture by
filtration at room temperature. It is worth to mention that,
when the complex Co-4 was stored under ambient conditions
before use, the catalytic activity remains and thus this catalyst
precursor seems to be air and moisture stable. (Table 1, entry
9).Delighted with these results, we employed catalyst Co-4 for
the isomerization of a variety of allylic alcohols, bearing aro-
matic or aliphatic substituents (Table 2). Noticeably, all termi-
nal allylic alcohols could be isomerized effortlessly. Isomeriza-
tion of substrates carrying internal double bonds was more
problematic. However, using trifluoroethanol (TFE) as solvent,
good to excellent yields of ketones could be produced with
these substrates (Table 2, products 2e, 2g, 2p). Generally,
fluorinated solvents display unique properties which are
known to be advantageous in many catalytic processes.[21]
Owing to the negative inductive effect of the fluorine substitu-
ent, the acidity of the hydroxyl group increases, hence a
strong hydrogen-bond-donating character results that can be
used as a promoter. We presume that chloride dissociation is
promoted in this solvent leading to a higher proportion of the
active catalyst (B in Figure 3). Interestingly, in marked con-
trast to the beneficial effect of TFE on the isomerization of
internal allylic alcohols, reaction of terminal substrates in this
solvent resulted in lower yields than in ethanol. Remarkably,
the slope of the conversion vs time curves reveal an initial
faster isomerization
Figure 2. Screening of cobalt catalysts for the isomerization of oct-1-en-3-ol
(1) to 3-octanone (2). Reaction conditions: 1.0 mmol substrate; 1.0 mL
ethanol; 3 mol% catalyst; 3 mol% KOtBu; 1 h. Determined by GC with n-
dodecane as internal standard.
Co-5 the oct-1-en-3-ol (1) stays untouched, excellent conver-
sions were achieved by using the less sterically hindered
complexes Co-1, Co-3, Co-4 and Co-6. Thereupon in the
second screening the most active and selective catalysts
towards the formation of 3-octanone (2) were tested in the
absence of base (Table 1, entries 1-3). Surprisingly, all cata-
lysts were still capable of performing the isomerization of 1 to
2. Notably, the conversions obtained with 1 mol% of Co-1 and
Co-6 at 120 °C dropped to 40% and 54% respectively, while
in the case of Co-4 only a slight decrease of the yield was
observed (see Table 1). The selectivity related to the desired
product 2 remains similar, compared to the reactions in the
presence of KOtBu. Nevertheless, Co-4 emerges to be the
best catalyst not only w.r.t. conversion of 1 but also w.r.t.
100% selective formation of 3-octanone (2) (Table 1, entry 3).
Table 2. Substrate scope of cobalt-catalyzed allylic alcohol isomerization[a]
Table 1. Cobalt-catalyzed isomerization of oct-1-en-3-ol (1). Optimization of
the reaction conditions.[a]
Entry
Catalyst
(mol%)
Temperature
[°C]
Conv. 1
[%][b]
2
3 [%][b]
[%][b]
1
Co-1 (1)
120
120
120
120
120
60
40
37
50
94
3
4
0
0
0
0
0
0
0
2
Co-6 (1)
Co-4 (1)
Co-4 (0.1)
Co-4 (0.1)
Co-4 (1)
Co-4 (1)
Co-4 (1)
Co-4 (1)
54
94
100
10
0
3
4[c]
5
>99
10
0
[a] 1.0 mmol substrate, 1.0 mL ethanol. Isolated yields. [b] Reaction time 8
h. cat: 0.1 mol%. 1.0 mL ethanol. [c] 1.0 mL of trifluoroethanol (TFE) instead
of ethanol. [d] Reaction time 4 h. [e] Reaction time 18 h. [f] GC-yield.
6
7
70
32
82
90
32
82
90
of 1 to 2 in TFE which then totally stops at a conversion of
85% (see ESI). As we suspected product inhibition, different
amounts of 3-octanone were added at the start of the reaction.
Indeed, the addition of 2 mmol of 3-octanone in TFE resulted
in complete loss of catalytic activity, confirming our suspicion.
8
80
9[d]
120
[a] 1.0 mmol substrate, 1.0 mL ethanol. [b] Determined by GC with n-
dodecane as internal standard. [c] Reaction time 8 h. [d] Co-4 was stored
24 h under aerobic conditions before using.
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CH(OD)-CHCH2) were prepared and both compounds were
subjected to the isomerization reaction (Scheme 1). From
these reactions the saturated ketones were obtained in which
the deuterium from the CD exclusively was found in the beta-
position and the deuterium from the OD in the alpha position.
This labelling pattern is in agreement with both a classical
alkene isomerization where the alkene inserts into the cobalt
hydride bond and beta-hydride elimination takes place from
the proton that is attached to the alcohol (Figure 3, left cycle).
But equally it is in agreement with the dehydrogenation, 1,4-
reduction mechanism that we had previously calculated to be
the correct mechanism for the iron-PNP-catalyzed allylic alco-
hol isomerization (Figure 3, right cycle).[5a]
Scheme 1 Deuteration experiments.
Different aryl substituted allylic alcohols bearing an aryl,
naphthyl, furanyl or thienyl group were investigated. These
substrates were converted to the corresponding ketones in
good to excellent isolated yields. Gratifyingly, the aliphatic
products (2c-2e, 2q) were isolated almost quantitatively even
at a lower catalyst loading of 0.1 mol%. To test the limitations
of the catalytic performance of Co-4, homoallylic alcohols (1l,
1m) as well as a cyclic allylic alcohol (1i) were examined.
While homoallylic alcohols could be still converted in moderate
yields, surprisingly, the cyclic substrate cis-verbenol (1i) forms
the ethyl ether. Presumably, due to the sterically demand of 1i
alkene insertion is hindered and HCl mediated allylic substitu-
tion becomes feasible.
Isomerization of primary allylic alcohols would lead to the
formation of an aldehyde.[22] Thus, cinnamyl alcohol was test-
ed but, unfortunately, only a 13% yield of aldehyde was ob-
tained after 18 h. This may be due to the progressive deacti-
vation of the active catalyst caused by the abstraction of
CO.[23]
Kinetics revealed that the reaction is first order in catalyst
and in allylic alcohol (see ESI, Figure 6), which again fits with
both mechanisms. However, when the reaction was stopped
after 10 min (20% conversion) and 30 min (69% conversion),
respectively, not a trace of -unsaturated ketone was found
in the crude mixture, which makes the redox mechanism less
likely. In order to discriminate between the two mechanisms
we performed a number of cross-over experiments (Scheme
2). First, 1-octen-3-ol was reacted in the presence of an α,β-
unsaturated ketone (Scheme 2, 2.1). If the reaction follows a
redox mechanism one would expect that the α,β-unsaturated
ketone would also be reduced to the saturated ketone. In
practice this compound is hardly reduced, which would argue
against the redox mechanism. However, the experiment is
rather biased as the terminal double bond of 1-octen-3-one is
probably 100 times more reactive than the internal double
bond of benzylidene acetone. To remove this bias we exam-
ined the reaction between 3-octanol and benzylidene acetone
(Scheme 2, 2.2). After overnight reaction, 30% of 3-octanone
and 10% of 4-phenyl-2-butanone had formed. Although it is
To obtain insight in the mechanism, the allylic alcohol 1a
was deuterated in two ways: Ph-CD(OH)-CHCH2 and Ph-
Figure 3. Proposed mechanisms for the cobalt catalyzed isomerization of allylic alcohols (dehydrogenation followed by either hydrogenation or hydride addi-
tion-elimination).
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clear that this establishes the possibility of a redox mecha-
nism it does not exclude the possibility of the double bond
isomerization mechanism. To obtain more conclusive evi-
dence, we decided to perform a cross-over experiment with a
terminal α, β-unsaturated ketone (Scheme 2, 2.3). Surprising-
ly, in this reaction the pent-1-ene-3-ol was not reacting but
instead an oxa-Michael addition of ethanol and a self-
condensation reaction presumably both mediated by the
catalyst, occurred. In addition, 6% of hydrogenated product
was detected caused by the slow transfer hydrogenation of
ethanol. We attribute the lack of reaction of pent-1-ene-3-ol,
which was rapidly converted to the ketone in 98% yield in the
absence of the enone, to the insertion of the more reactive
CC bond of the enone into the cobalt hydride bond. The re-
sulting cobalt alkyl complex is stabilized by the presence of
the carbonyl group and is likely to be the resting state. Given
the fact that 1-phenyl-2-propen-1-ol was smoothly isomerized
to butyrophenone (2a) without any trace of the oxa-Michael
adduct, this experiment makes the redox mechanism highly
which has an ethyl group on the nitrogen and no N-Co bond,
was a very poor catalyst in the isomerization of 3-octen-1-ol in
ethanol (2.4). We assume that Co-4a is inactive since it can
no longer form a cobalt hydride.
In summary, we tested a series of cobalt PNP-pincer
complexes in the isomerization of allylic alcohols to the ke-
tones. In particular, the versatile complex Co-4 has been
found to be the most efficient catalyst which surprisingly func-
tions well without the addition of base. A range of allylic alco-
hols bearing aliphatic or aromatic groups were successfully
converted. It was shown that the use of catalytic amounts of
base speeds up the reaction, presumably by increasing the
amount of active complex B.
unlikely. Thus
a hydride addition-elimination mechanism
would seem highly likely. To obtain further proof, allyl benzene
was subjected to an alkene isomerization with Co-4. When a
non-protic solvent like acetonitrile, THF or toluene was used,
no isomerization product was formed (Scheme 2, 2.5) where-
as using ethanol as solvent, allyl benzene was almost quanti-
tatively isomerized to 1-phenyl-1-propene (Scheme 2, 2.6).
This confirms that the cobalt hydride that is formed by dehy-
drogenation of ethanol is capable of alkene isomerization via
alkene insertion elimination.
Scheme 2. Reactions for mechanistic insight.
Figure 4. Successive dehydrogenation and condensation of ethanol by Co-
4 leads to the formation of Co-4a. ORTEP representation of Co-4a (30%
probability ellipsoids, hydrogen atoms are omitted for clarity).
Allylic alcohols containing internal double bonds were convert-
ed much slower, but these reactions could be accelerated by
using trifluoroethanol as solvent. We propose a mechanism in
which cobalt retains its (II) oxidations state. The active cata-
lyst is formed by dissociation of HCl followed by dehydrogena-
tion of an alcohol to form a cobalt hydride complex, which can
isomerize the allylic alcohols by a classical alkene insertion/
elimination mechanism.
Thus, we propose the following mechanism. The activa-
tion of the cobalt complex takes place by loss of HCl. This is
accelerated by the use of base, but we assume that without
base there is equilibrium between Co-4 and species B in polar
solvents. In toluene the equilibrium is completely on the side
of Co-4 explaining the lack of isomerization of allylbenzene in
toluene. Next, either the allylic alcohol or ethanol is dehydro-
genated by B in an outer sphere fashion to form the hydride
species C. Insertion of the alkene into the cobalt hydride bond
is possibly preceded by dissociation of chloride, which may
bind again after the insertion of the alkene. Likewise, dissocia-
tion of the chloride is likely preceding the beta-hydride elimi-
nation, which leads to formation of the enol which tautomeriz-
es to the ketone. Although the competing redox mechanism
would seem to be much slower, it cannot be completely ex-
cluded.
Acknowledgements
We thank the analytical department of LIKAT for their indis-
pensable support. We would like to thank Arend Rösel and
Robert Francke for cyclic voltammetry characterization of the
catalyst. We are thankful to the support of Martyna Wozniak
for the graphical abstract. Tian Xia is grateful for the financial
support of the Chinese Scholarship Council (CSC). This work
was also supported by the state of Mecklenburg-Vorpommern
Interestingly, if complex Co-4 is stirred in ethanol over-
night at 120 °C, we were able to isolate complex Co-4a. We
assume this has formed from the dehydrogenation of ethanol
to acetaldehyde, which reacts with the nitrogen of the ligand in
either Co-4 or complex B followed by reduction of the ensuing
imine/enamine. While Co-4 is a highly active catalyst, Co-4a,
Keywords: Isomerization • Allylic alcohol • Cobalt catalyst • Pincer ligand •
Ketone
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10.1002/chem.201901148
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
B. Spiegelberg, A. Dell´Acqua, T. Xia, A.
Spannenberg, S. Tin, S. Hinze, J. G. de
Vries*
Page No. – Page No.
Additive Free Isomerization of Allylic
Alcohols to Ketones with a Cobalt
PNP Pincer Catalyst
A powerful cobalt drive. Cobalt PNP pincer complexes were found to be efficient catalysts for the isomerization of allylic alcohols to
the ketones. The catalyst with phenyl groups on the phosphorus atoms was highly active in the absence of base. A broad variety of
substrates were successfully converted. The mechanism proceeds via alkene insertion in the cobalt-hydride bond followed by beta-
hydride elimination.
This article is protected by copyright. All rights reserved.